These well-loved tunes have been around for a long time. Perhaps the oldest is Nobody Knows the Trouble I've Seen, a Negro spiritual that may have been written by slaves. Others come from the latter half of the 19th century and from the turn-of-the-century era that produced the precursors of jazz. The unifying factor in selecting these particular tunes, apart from the fact that every American seems to know them, is that they have been constantly recreated by jazz performers, from the birth of jazz to the present day. There is an immediate link from the improviser to the mood, structure, or chord structure (sometimes all three!) of these tunes that allows for a free-flowing personal variation on these timeless themes.